A disc chipper is commonly used in the paper and pulp industry for chipping wood for pulping. Aligned substantially radially on the knife disc of a disc chipper are mounted knives that against a stationary anvil knife cut wood into chips. The sectors remaining between the knives are covered with wear plates against which the wood to be chipped are fed. As the entering logs impose a continuous abrasion on the wear plates, they must be made sufficiently strong against continuous wear. A disc chipper and its wear plates is described in patent application publication WO 96/26817.
To make the machining of the wear plates easy, they are conventionally fabricated from a relatively soft steel grade and then coated with a wear-resistant material on their front side. The hardness and wear resistance properties of the coating are selected to be substantially higher than that of the wear plate base material, whereby the coating forms a stronger surface against wear. Conventionally, the coating is applied as a powder that is sprayed and melted (sintered) in a heat treatment process on the surface of the wear plate front side. The melting step takes place in an oven at a temperature in excess of 1000° C. During the heat treatment process, the wear plate blank typically undergoes an unpredictable amount of minor deformations. Hence, the uncoated surfaces of the wear plate are post-machined to correct dimensions only after the heat treatment.
Depending on the chipped quantity, species of wood and amount of impurities carried over therewith, hardfaced wear plates last 6 to 36 months of use. Certain tropical deciduous wood species in particular may reduce the life of hardfacing on the wear plate even shorter than six months.
Generally, a single knife disc may contain 10 to 16 wear plates. Since the replacement of wear plates with entirely new ones is relatively expensive, users of disc chippers strive to recondition outworn wear plates for reuse. A problem hampering rehardfacing is the deformation of wear plate dimensions due to the heat treatment required in the coating process. However, a wear plate to be reconditioned with a new hardcoating on its wear surface does not have any machining allowance for correcting dimensional errors. If shape deformations are machined away, the shape of the wear plate falls short of the acceptable minimum dimensions, whereby it cannot be used as a replacement in conjunction with new wear plates. Hence, all wear plates of a disc chipper must be replaced at the same time if reconditioned wear plates are installed. Furthermore, great care must be taken to avoid mixing reconditioned wear plates with new wear plates at any time.